This invention relates to recovery of carbonaceous materials from underground deposits. More specifically, this invention relates to the subsurface combustion and retorting of hydrocarbonaceous materials such as oil shale.
Numerous hydrocarbonaceous materials are found in underground deposits; for example crude oil, coal, shale, oil, tar sands, and others. One method of recovering energy or hydrocarbon from such underground deposits is by underground combustion. An oxidizing gas such as air can be provided to an underground combustion or retorting zone so as to combust a portion of the combustible material contained therein and free hydrocarbon or thereby form materials which are suitable for energy recovery. For example, air or oxygen, and diluent gases such as steam, can be passed into a coal deposit so as to form off-gases having combustible materials such as light hydrocarbons and carbon monoxide. These gases can then be combusted directly for heat, or energy recovered such as through power generation. Underground combustion can be used in the recovery of petroleum crude oil from certain types of deposits. Air or oxygen, and steam, is passed into an underground deposit and combustion initiated so hot combustion gases will aid in the recovery of such crude oil. Similar technique can be used in the recovery of oil from tar sands. One important use of underground combustion is in the recovery of oil from oil shale.
The term "oil shale" refers to sedimentary deposits containing organic materials which can be converted to shale oil. Oil shale can be found in various places throughout the world, especially in the United States in Colorado, Utah, and Wyoming. Some especially important deposits can be found in the Green River formation in the Piceance Basin, Garfield and Rio Blanco counties, in Northwestern Colorado.
Oil shale contains organic material called kerogen which is a solid carbonaceous material from which shale oil can be produced. Commonly oil shale deposits have variable richness or kerogen content, the oil shale generally being stratified in horizontal layers. Upon heating oil shale to a sufficient temperature, kerogen is decomposed and liquids and gases are formed. Oil shale can be retorted to form a hydrocarbon liquid either by in situ or surface retorting. In surface retorting, oil shale is mined from the ground, brought to the surface, and placed in vessels where it is contacted with hot retorting materials, such as hot shale or gases, for heat transfer. The hot retorting solids or gases cause shale oil to be freed from the rock. Spent retorted oil shale which has been depleted in kerogen is removed from the reactor and discarded. Some well known methods of surface retorting are the Tosco, Lurgi, and Paraho processes and fluid bed retorting.
Another method of retorting oil shale is the in situ process. In situ retorting of oil shale generally comprises forming a retort or retorting zone underground, preferably within the oil shale zone. The retorting zone can be formed by mining an access tunnel to or near the retorting zone and then removing a portion of the oil shale deposit by conventional mining techniques. About 2 to about 45 percent, preferably about 15 to about 40 percent, of the oil shale in the retorting area is removed to provide void space in the retorting area. The oil shale in the retorting area is than rubblized by well-known mining and blasting techniques to provide a retort containing rubblized shale for retorting. In some cases it is possible to rubblize underground oil shale without removal of a portion of the oil shale. However, it is generally preferable to remove material so as to provide void space which will result in more uniform rubblization and more efficient use of explosives.
A common method for forming the underground retort is to undercut the deposit to be retorted and remove a portion of the deposit to provide void space. Explosives are then placed in the overlying or surrounding oil shale. These explosives are used to rubblize the shale, preferably forming a zone of rubble having uniform particle size and void spaces. Some of the techniques used for forming the undercut area and the rubblized area are room and pillar mining, sublevel caving, crater retreat and the like. Because of the stratification of oil shale it may be desirable to selectively mine material based on its mineral or kerogen content for removal from the retorting zone. Also because of the stratification, the retorting zone may contain lean oil shale, or rock containing essentially no kerogen. After the underground retort is formed, the pile of rubblized shale is subjected to retorting. Hot retorting gases are passed through the rubblized shale to effectively form and recover liquid hydrocarbon from the oil shale. This can be done by passing a gas comprising air or air mixed with steam through the deposit. Air can be forced into one end of the retort and a fire or flame front initiated. Combustion can be initiated by introducing fuels such as natural gas, propane, shale oil, and the like which are readily combustible with air. After combustion has been initiated, it can be sustained by combusting coke on spent or partially spent oil shale, oxygen contacting the coke forming or maintaining a flame front. This flame front is then passed slowly through the rubblized deposit to effect retorting. Actually the hot combustion gases passing ahead of the flame front cause the retorting of oil shale and the formation of shale oil. Another suitable retorting fluid comprises hot combustion or retorting off-gas from the same or nearby underground retort. Not only is shale oil effectively produced, but also a mixture of off-gases is produced during retorting. These gases contain hydrogen, carbon monoxide, ammonia, carbon dioxide, hydrogen sulfide, carbonyl sulfide, oxides of sulfur and nitrogen, and low molecular weight hydrocarbons. Generally a mixture of off-gases, water and shale oil are recovered from the retort. This mixture undergoes preliminary separation commonly by gravity to separate the gases from the liquid oil from the liquid water. The off-gases commonly also contain entrained dust, and hydrocarbons, some of which are liquid or liquefiable under moderate pressure. The off-gases commonly have a very low heat content, generally about 50 to about 150 BTU per cubic foot.
A number of patents describe methods of in situ retorting of oil shale, such as Karrick, L. C., U.S. Pat. No. 1,913,395; Karrick, S. N., U.S. Pat. No. 1,919,636; Uren, U.S. Pat. No. 2,481,051; Van Poollen, U.S. Pat. No. 3,001,776; Ellington, U.S. Pat. No. 3,586,377; Prats, U.S. Pat. No. 3,434,757; Garrett, U.S. Pat. No. 3,661,423; Ridley, U.S. Pat. No. 3,951,456; and Lewis, U.S. Pat. No. 4,017,119 which are hereby incorporated by reference and made a part hereof. These references teach both up-flow and down-flow vertical retorts. Both up-flow and down-flow retorting is discussed in Liquid Product From Bottom Burn Shale Retort, Richard C. Aiken, University of Utah (1979).
One problem in the underground combustion and retorting of carbonaceous materials such as shale oil deposits is the difficulty in forming and maintaining a uniformly oriented or even flame front. If a portion of the flame front advances more quickly than other portions, large portions of the rubblized matter will be bypassed and will not be effectively retorted, thereby diminishing overall recovery of energy from the deposit. This is partially attributable to the difficulty in forming a uniform rubblized mass with uniform gas passages, and also uniformly passing gas into and out of the retorting area. If a narrow portion of the flame front advances completely through the retorting area, high temperature and/or oxidizing gas which is passed into one end of the retort will eventually break through the flame front at the leading position and pass to the off-gas collection system (breakthrough). This will overload the off-gas collection system with oxidizing gas which has not had an opportunity to partake in the combustion process. Therefore, flame front breakthrough can lead to the termination of retorting of an oil shale retort before all of, or even a substantial portion of, the rubblized mass of oil shale is retorted, thereby lowering energy recovery from a retort. Flame front breakthrough can also be dangerous because it can result in a combustible or explosive gas composition in the product recovery zone.
It is an object of this invention to provide a process for the efficient recovery of energy from underground deposits of hydrocarbon so that higher yields of energy can be recovered from a given deposit.
It is an object of this invention to prevent the overloading of off-gas recovery systems attendant to underground combustion processes and to prevent dangerous gas compositions in off-gas recovery systems.
It is an object of this invention to retort substantially all of the rubblized oil shale within a retort, thereby maximizing energy recovery.